Audio engineers have long recognized that conventional analog recording techniques are rapidly approaching theoretical operational limits, thus leaving little room for further significant improvement of high quality sound reproduction by these methods. On the other hand, proposals for the use of digital signal handling techniques offer rewarding alternatives because of inherent theoretical advantages. For one thing, the signal to noise ratio for digital signals remains almost entirely dependent upon the accuracy of the initial conversion and, unlike analog signals, the digital signal is thus largely unaffected by the amount of further handling. Moreover output signal level is not dependent on gain stability of the various circuits and channels, and problems of frequency dependent phase shift or other nonlinearities are not encountered during transmission. Digital signals can also be delayed or stored on magnetic media for any length of time without degradation of the recording due to "print-through" from the interaction between adjacent layers of tape or demagnetization. Futhermore, no degradation of signal to noise ratios occurs due to copying or problems of cross-talk between channels, and tape motion problems involving flutter and wow effects can be eliminated with simple digital buffers.
But serious problems have arisen in the practical application of digital techniques to audio signals. First of all, poor transmission conditions that would normally only degrade an analog signal can destroy its digital equivalent, and even a small discontinuity can produce very unpleasant audio disturbances. Even a single bit error, if it occurs in the most significant digit positions, can produce sudden drastic changes in signal output level of up to one-half full scale that cause very loud and unpleasant clicking noises.
To minimize the effects of data error, much effort has been expended in devising and testing various complex data recording and transmission formats. High performance data processing equipment and techniques currently available are much too expensive even for commercial audio systems, and numerous difficulties had been encountered in achieving the required reliability within the capabilities and price range of existing professional audio tape transport systems. The principal constraint lies in the high data bit packing densities needed to handle required sampling frequencies in the order of forty kiloHertz while providing sufficient quantizing bits to achieve significantly improved signal to noise ratios at conventional audio tape speeds. The usual expediency of using parallel track recording to achieve greater bit packing densities only introduces system complexities associated with tape skew and data synchronization. Multiple audio channels further complicate the situation.
A summary of recent developments and trends in the mechanization of digital audio systems can be found in the article by J. Dwyer entitled "Digital Techniques in Recording and Broadcasting" published in the June 1975 issue of "Wireless World" . The approach suggested therein generally involves the use of a logarithmic type quantizing scale with interleaved multiple track recording of data words containing parity bits for detecting data error in the most significant bits. When data error is detected, the output is simply held at its previous correct level to minimize audible discontinuity. But, this approach has severe limitations if the actual loss of signal or persistent error is extended over more than a few sampling intervals. In that case, the audible discontinuity would become quite noticeable and the loud "click" noise would be quite evident where the signal level changed significantly during the interim.